I was planning a lesson on conditional sentences and I came across a site that talked about types of conditional statement one of which is 'zero conditional.' It says this kind of conditional statement is used to express general truths or scientific facts in which one thing causes another. It also says that the verbs in both the conditional clause and the main clause in the statement should be simple present tense, e.g.
1. If people smoke cigarettes, their health suffers.
Then, it says it is wrong to use the future 'will' in such a statement, e.g.
2. If people smoke cigarettes, their health will suffer. (Considered wrong)
I am confused by that rule that using 'will' makes the sentence incorrect. Unfortunately, none of the grammar texts I have discuss anything like 'zero conditional'.
I'd like to know if the rule is true and if there are standard grammar texts to support it.
Also, which of the following pair of examples is correct?
3a. If people go permanently blind, they will not see again.
3b. If people go permanently blind, they don't see again.
4a. If you leave yam tubers in the soil for days, they will rot.
4b. If you leave yam tubers in the soil for days, they rot.
Thanks so much. This page has been of great help to me since I was in university and now that I am teaching.